The present invention relates to electronic article surveillance systems and more specifically, to electronic article surveillance systems which detects markers in an interrogation zone
Electronic article surveillance (EAS) systems of the magnetic type are extensively described in the art. In such systems, a magnetic marker having a specific non-linear response is attached to an article under surveillance. An alternating magnetic field is generated by an antennae system in an interrogation zone. If an article carrying a marker passes through the zone, perturbations are created in the magnetic field. These perturbations are sensed by receiver antenna coils, and the corresponding electrical signals are analyzed by a detector unit. An alarm is activated in response to particular signal pattern.
Several methods for article surveillance have been described. One such system uses magnetic markers having a hysteresis loop with large Barkhausen discontinuities. Such systems are available from Sensormatic Electronics Co. of Boca Raton, Fla., USA. The operation of a typical system operating under this principle is disclosed in U.S. Pat. No. 4,859,991 to Watkins et al. and the patents cited therein. Since the marker exhibits a step function reversal of its magnetization when exposed to a low frequency interrogation magnetic field above a certain threshold, the perturbations of the received signal are rich in high order (about 20 to 100) harmonics that can be easily distinguished from signals generated by other ferrous objects.
While this system provides detection capabilities, it requires relatively large marker sizes (over 45 mm, and typically 90 mm length) to provide reliable detection. Use of high order harmonics for detection results in relatively low sensitivity, as only a small portion of the marker response energy is available at such harmonics. Consequently, in practical use the aisle width in the interrogation zone is decreased to 80 cm or less, depending on the noise level in the environment.
Esselte Meto GMbH, of Heppenheim, Germany, produces another system type. This system uses amorphous magnetic markers characterized by very low coercivity and high permeability. Variants of such system embodiments are disclosed in U.S. Pat. No. 5,414,410 to Davies et al. and EP 0153286 to Esselte Meto EAS International AB. Generally, transmitting antennae generate magnetic fields of two or three different frequencies, and the marker nonlinear response results in intermodulation products of these frequencies that are detected by the signal processing unit. The Meto systems feature high sensitivity at rather wide aisles (up to 120 cm) and relatively small markers (32 mm typical length).
Both systems exhibit poor detection characteristics when the marker is passed in a plane parallel to antennae or within a small angle (20-30 degrees) to that plane. Another common problem is poor or no detection of markers attached to highly conductive objects (aluminum, copper), as relatively high frequency (10-12 KHz) intermodulation response signals of the marker are suppressed by induced eddy currents in the conductive object.
Intrinsic disadvantage in the above described systems are low or zero sensitivity zones, commonly referred to as xe2x80x98dead zonesxe2x80x99, exhibited within the detection zones. Typically, the receiving antennae coils in such systems are constructed in figure eight shape. Generally, dead zones are present in the area near the intersection of the figure eight shape. Attempts to eliminate this disadvantage by more sophisticated configurations of the receiver antenna coils like those described in U.S. Pat. No. 5,459,451 to crossfield et al. result in cumbersome and expensive antenna structures. Purinton et al. in U.S. Pat. No. 3,990,065, attempted to increase detection by varying the spatial orientation of the magnetic field by increasing the number of the antennae coils, also resulting in cumbersome and expensive antenna structures.
Recent trends in prevention of shoplifting are towards the use of small, thin and flexible markers at maximum attainable aisle width, which requires the highest system sensitivity in the harsh interference conditions of the point of sale. Also, fast growth of source tagging technologies makes it desirable for an EAS system to be operable with different marker types, be those of a harmonic type (like those of Meto) or Barkhausen type (like those of Sensormatic), or any other applicable marker.
It is an aim of the present invention to provide an electronic article surveillance system with improved design that allows highly reliable detection of markers in any orientation, and along any trajectory within the interrogation zone.
Amongst other parameters, the preferred embodiment of the invention achieves better detection capabilities providing excitation signals in a plurality of polarization planes, while requiring only a minimum of four transmitting antennae coils. By providing excitation in multiple planes, the marker almost always receives sufficient energy for activation regardless of its orientation. The magnetic field orientation is varied by simultaneously feeding current to the antennae coils in varying polarities. Varying polarity patterns, or phase patterns, are selected to cause different orientations of the magnetic field due to field interaction between the fields generated by any two or more antennae coils.
Thus, in a broad aspect of the invention two substantially parallel antennae arrays are provided, forming an interrogation zone between the arrays. Each array comprising at least two substantially coplanar antenna elements, each having at least one transmitting coil. A phase sequencer is adapted to feed power to the antennae coils in varying phase patterns and is coupled to them. The patterns are selected to produce magnetic field of different spatial orientations, by magnetic field interaction between the magnetic fields generated by at least one pair of antenna coils. Thus at least one of the patterns is selected to cause the instantaneous current in a first coil to flow in an opposite direction to the instantaneous current in a second coil, so as to generate a field induced by the first coil, that is of different spatial orientation than the field of the second coil, and the interaction therebetween causes a magnetic field of a third orientation. The phase sequencer switches between the phase patterns, in a time dependent fashion.
The term phasing as used in this application relates to the relationship between the orientation of the magnetic field generated by a first coil and the orientation of the magnetic field generated by a second coil, or to the currents that cause such magnetic fields. The product of the interaction between a plurality of field is the result of phasing and may take different spatial orientation from the first and the second field.
In accordance with the preferred embodiment of the invention, the different spatial orientation of the magnetic field are provided by using a plurality of transmitting coils wherein the phases of alternating current in the coils are switched in accordance with the pre-determined timing sequence.
In the preferred embodiment, at least one of the antennae elements, and more preferably each of the antenna elements also comprise a receiving coil. In the most preferred embodiment, each antenna element has also a compensator coil, located at closer proximity to the transmitting coil than the proximity of the receiving coil and having fewer turns then those of the receiving coil, each receiving coil coupled to a corresponding com[pensating coil. The coupling between the coils is done in opposite polarity, so that the voltage induced by the transmitting coil in one coil will be substantially neutralized by the voltage induced in the other. The coupled coils form a receiving element. The receiving elements, or in a minimal embodiment only the receiving antennae, are coupled to a receiver and a signal processor, which in turn analyses the signals received by the antenna and determines the presence of a marker in the interrogation zone. The receiver and signal processor may be integrated.
Optionally, digital signal processing is used to improve detection and avoid false alarms. In this digital processing, a sliding window in combination with a pre-determined model of the expected marker response, is utilized to determine if a received response was generated by the presence of a marker in the interrogation zone.
Further disclosed is a method for detecting the presence of a marker within an interrogation zone, the method comprising the steps of feeding current to a plurality of transmitting coils in varying phase patterns. A first pair of transmitting coils are arranged in a first antenna array, and a second pair of transmitting coils are arranged in a second antenna array, substantially parallel to said first antenna array. The two arrays form an interrogation zone therebetween. The phase patterns are selected to cause a different spatial orientation of the magnetic field in the interrogation zone for specific different phase patterns. Modifying said phase patterns in a time dependent manner. Sensing magnetic perturbations caused by the presence of a marker in the interrogation zone, analyzing signals resulting from said sensing; and, outputting an indication if said step of analysis determines that a marker is present within the interrogation zone.
Preferably, the step of sensing is performed utilizing a receiving coil and a compensating coil located in the antennae array, wherein the output of said receiving coil is coupled to the output of said compensating coil at opposite polarity. The preferred construction of the receiving antenna elements was provided elsewhere in these specifications. More preferably the step of sensing is performed using receiver and signal processor, which may be integrated. Most preferably the method also comprises the step of comparing the received magnetic field perturbations to a pre-determined model of the expected marker response.